Interior rotation, mixing, and ages of a sample of Slowly Pulsating B stars from gravitymode asteroseismology

The study of stars forms the basis of many research fields in astrophysics, from exoplanet systems to the formation and evolution of galaxies. Stars are the chemical factories of the Universe, responsible for the production of heavier elements in their cores throughout their evolution. These resulting products of the internal nucleosynthesis are fed back to the interstellar medium through stellar winds, which chemically enrich the stellar environment. If not for these processes, life as we know it on Earth would never have come to exist. Massive stars in particular play an important role in the chemical enrichment of the universe, as they lose a considerable amount of their mass through their strong stellar winds throughout their evolution. Sufficiently massive stars end their lives with supernovae explosions which further chemically enriches the Universe through the production of even heavier elements.

How exactly this happens is described by our stellar structure and evolution models. Any uncertainties in the input physics of these models will propagate throughout all fields of research in astrophysics, which relies on the understanding of stars. Therefore, obtaining good stellar structure and evolution models is one of the basic requirements of astrophysics.

Throughout the majority of their evolution, as they fuse hydrogen to helium in their cores, massive stars (as opposed to low-mass stars) have a convective core. The convection causes the material in the core to be fully mixed, meaning that all of the material inside the convective region can be used as fuel in the fusion process. Therefore, the size of the core heavily influences the lifetime of massive stars. Due to the nature of convection, the boundary between the core and the radiative envelope is not sharp. Instead, hot blobs of gas will overshoot into the radiative region before coming to a stop and falling back to the core. This effectively increases the size of the convective core and thereby the lifetime of massive stars. The extend and shape of this overshooting is poorly understood and therefore provide a large uncertainty in stellar modeling. Other mechanisms which lead to internal mixing in stars are, e.g., the presence of magnetic fields and internal rotation. The influence of these mechanisms is approximated and described by a diffusion coefficient.

Asteroseismology provides the tools we need to reach a better understanding of these mixing processes. Previously, the vast amount of newly available data from space missions have made asteroseismology an observationally driven field of research, and the main focus has been on obtaining high-precision stellar parameters needed for, e.g., exoplanet studies as well as input for further stellar and galactic studies. However, now the time has come for testing the input physics for the stellar structure and evolution models by comparing model predictions to observations.

Just as seismology has provided information on the internal structure of the Earth, stellar oscillations probe the stellar interiors and carry with them clues about the structure beneath the stellar surface. Gravity modes (g-modes) in particular have considerable amplitudes throughout the entire interior of the star, including the narrow overshoot range around the convective core. Therefore, by studying these observed modes and comparing them to theoretical predictions, we can constrain the physical properties and extend of the overshoot region. This process is referred to as forward seismic modeling. The number of stars for which such modeling has been possible is very limited, emphasizing the importance of focusing on such studies in the future.

The aim of this PhD project is to perform detailed forward seismic modeling of single massive stars, in order to test the input physics of the stellar models and to reach a better understanding on the internal mixing processes operating in these stars.